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  <title>Document</title>
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<body>
  <script>
    var arr = [1, 3, 5, 7, 9]

    var str = ''
    var flag = ','

    // i   index  索引
    for(var i = 0; i < arr.length; i++) {
      if(i === arr.length - 1) {
        str += arr[i]
      } else {
        str += arr[i] + flag
      }
    }

    console.log(str)

  </script>
</body>
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